North Korean media announced on Friday that Jang Song Thaek had been killed after a special military trial.

We would urge the North Koreans not to take provocative acts, not to do so going forward because it is not in interest of regional stability.

Marie Harf, US State Department spokeswoman

He was described as "human scum" and "worse than a dog" for allegedly committing crimes not detailed but for which was said to have pleaded guilty.

China, South Korea, Japan and the US are closely monitoring the situation, which is the biggest political upheaval since Kim Jong-Un took power and may cause significant instability.

US State Department spokeswoman Marie Harf would not speculate on what might be happening within the North Korean regime, but said the execution was a "brutal act" which "underscores the horrific human rights record of the North Korean regime".

"We would urge the North Koreans not to take provocative acts, not to do so going forward because it is not in interest of regional stability," she said..

The US is already in regular talks with Beijing, a close ally of Pyongyang, and Ms Harf says it is consulting its allies in the region to draw conclusions on what took place and the impossible implications.

"What we really see is that the North Korea has a choice right between continuing down the pass of isolation and impoverishment of its own people or meeting its obligations and coming back to the international system," Ms Harf said.

"That is certainly what we have talked to the Chinese and others about and the point that Chinese are making to the North Koreans directly as well."

South Korea has also expressed its "deep concern" after ministers related to security met to discuss the execution.

"The government has deep concerns about a recent series of developments in North Korea and is watching the situation closely," the South's Unification Ministry said in a statement.

It says South Korea will prepare for "all possibilities in the future" in close collaboration with its allies.